The present invention relates to a mask for screening conductive patterns on substrates, and more particularly, relates to a mask and screening method for double pass screening of conductive patterns on substrates.
In the fabrication of packages for semiconductor devices and the like, it is necessary to deposit finely detailed patterns of conductive material on insulative substrates such as those made from ceramic materials. In order to provide these conductive patterns, conductive paste is deposited on the insulative substrate through a screening mask.
Screening masks are typically made of a metallic material such as molybdenum, copper, nickel and the like. However, the material of the screening mask is not important to the present invention.
The conductive patterns typically consist of wiring lines and vias. In one method of screening, the wiring lines and vias are deposited at the same time through a single screening mask. This method of screening is called single pass
In another method of screening, the wiring lines and vias are deposited at two different times through two different screening masks. This method of screening is called double pass screening. That is, the first pass fills the vias, and sometimes also deposits a via cap, while the second pass deposits the surface wiring lines. The greensheets are typically dried between screening passes which may lead to some distortion in the greensheets so that the first-deposited pattern is slightly off from ideal. Then, when the mask for the second pass is applied, the mask can cause damage to the first-deposited pattern. This damage can cause poor yields due to shorts or extraneous metal.
Utter et al. U.S. Pat. No. 5,819,652 discloses a conventional screening mask for screening wiring lines and vias in a single pass.
Conventional double pass screening is disclosed in Inasaka U.S. Pat. No. 5,792,293, the disclosure of which is incorporated by reference herein. As shown in FIGS. 6 and 7 of Inasaka, vias are first screened followed by screening of the conductor pattern. This reference does not disclose the masks used during the first and second screening passes.
A conventional screening mask is disclosed in Comino et al. U.S. Pat. No. 6,095,041, the disclosure of which is incorporated by reference herein. As can be seen in FIG. 1 of the reference, the screening mask is principally open for screening of features except for tabs bridging the open features so as to provide structural rigidity to the screening mask.
Notwithstanding these advances in the art, there remains a need for a screening mask and screening method suitable for double pass screening which will not damage a previously deposited pattern.
Accordingly, it is a purpose of the present invention to have a screening mask and screening method for double pass screening which will not damage a previously deposited pattern.
It is another purpose of the present invention to have a screening mask for double pass screening that can be easily manufactured and a screening method for double pass screening that can be easily implemented.
These and other purposes of the present invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.
The purposes of the present invention have been achieved by providing, according to a first aspect of the present invention, a method of double-pass screening an article, the method comprising the steps of:
applying a first mask and screening through the first mask a first feature with respect to the article;
removing the first mask; and
applying a second mask and screening through the second mask a second feature with respect to the article, the second mask having no opening directly over the first feature.
According to a second aspect of the invention, there is provided a method of double-pass screening a ceramic greensheet, the method comprising the steps of:
applying a first mask and screening through the first mask at least one via in the ceramic greensheet; removing the first mask; and
applying a second mask and screening through the second mask at least one wiring line on the ceramic greensheet, the second mask having no opening directly over the at least one via.
According to a third aspect of the invention, there is provided a screening mask for screening an article, the screening mask comprising:
a screening mask body having a screening side and a nonscreening side;
at least one opening through the mask for screening at least one feature from the screening side to the nonscreening side; and
at least one blind opening for protecting a previously screened featured, the blind opening blocked on the screening side and open on the nonscreening side.